Nobody knows. That is the honest headline, and it is worth saying at the top, because the straightness of a QCW's sword depends on the room more than it depends on the coil, and two people with the same machine get different results and go looking for a fault that is not there.
This is a collection of what has been observed and of the guesses attached to it. Where the data stops and the speculation starts is marked.
Two defects, constantly confused
Branching is the arc growing in all directions at once. Wander is the arc curving as a whole, sometimes right round into a loop.
They respond to frequency in opposite directions. Raise the frequency and branching gets better, because the channel runs hotter. Raise it and wander gets worse: a ramped VTTC at 2 MHz makes loops tens of centimetres across.
A straight arc is the absence of both, which is why there is an optimum around 200 to 500 kHz rather than a rule that higher is better.
What people have seen
CJ has the most detailed set. Moved the machine to a different spot and the arc bent. Put it near a wooden cabinet and it straightened. Cold winter, bending down towards the ground; lit the stove, it warmed up, it straightened. Close to an iron frame, straighter. The thickness of the grading ring barely mattered.
Jan, after fifteen years of building coils, met this for the first time: arcs looping downward with a radius under a metre, striking the base of the coil itself, sometimes in an S. Dry air made them straighter. Temperature by itself did almost nothing. Opening the garage door to about ten degrees of outside air reduced the looping.
paulsimik saw arcs curl almost to the floor at low atmospheric pressure, with temperature and humidity unchanged.
Four hypotheses, none of them settled
The arc follows the toroid's field lines. Uspring's. A QCW arc is slow enough to follow the field. A small toroid holds little charge, so its influence falls away quickly with length, and the arc leaves for the nearest ground: floor, wall, ceiling. Outdoors there is no ceiling, so the loop goes to the ground or a wall.
The arc as a distributed RC. Uspring again, and he marks it as speculation. A phase shift builds up between the charge on the toroid and the charge at the tip. As it approaches 180 degrees the tip starts being attracted back towards the toroid, and you get a loop. A distributed RC really can produce any shift you like, but with attenuation, and that part is not worked out. It predicts the effect getting worse with frequency, and the 2 MHz VTTC agrees.
Uniformity rather than absolute values. davekni has no conclusion; his best guess is that what matters is how uniform the temperature and humidity are rather than what they are. He also notes that wood conducts at megahertz and that dielectrics matter, which fits CJ's wooden cabinet.
Humidity through channel cooling. Mads: a warm room means lower relative humidity, so drier air, so the arc's attachment to ground behaves differently. Separately, humidity affects how the channel is cooled and how much energy goes into evaporating water.
What the data does not support
There is no single factor. Temperature worked for CJ and did almost nothing for Jan. Pressure worked for paulsimik and nobody else has tested it.
Almost none of this is a direct experiment. It is nearly all "I changed the surroundings and it behaved differently", where several things changed at once.
What to do with it anyway
- If your arc bent after a move or a cold snap, look at the room before you touch the tuning. That is the most useful thing in this whole piece.
- Do not compare your results to somebody's video without knowing the conditions it was shot in.
- If it loops down towards the base, try a bigger toroid. That is hypothesis one, untested.
- Check whether it is actually branching rather than wandering. Branching has its own causes: the lower pole, too fast a ramp, a corner in the ramp, or residual voltage left on the bus.
What would close it
- One factor isolated. One coil, one room, change only the humidity, or only the pressure, film it on one camera.
- The toroid test. Two toroids of different capacitance, everything else held.
- The frequency dependence on one machine, rather than by comparing different ones.
- Numbers. Every observation above is qualitative. Nobody has any figures at all.
Whoever collects even one of those measurements closes a question the community has not managed to close.
The QCW department diagram draws its arc straight, which is the good case rather than the ordinary one. The records page collects the builds that got there.